So say researchers who mapped the destructive path from smoking to the debilitating lung disease in mice.

"Previously, emphysema was thought to be a nonspecific injurious response to long-term smoke exposure," study author Dr. Farrah Kheradmand, a professor of medicine and immunology at Baylor College of Medicine, said in a college news release. "These studies show for the first time that emphysema is caused by a specific immune response induced by smoke."

She and her colleagues spent more than four years unraveling how smoking leads to emphysema. They exposed mice to conditions that closely simulated how humans smoke. The mice developed emphysema within three to four months, and certain inflammatory cells and genes were crucial in the process, Kheradmand said.

"It is a combination of little genes affected by an epigenetic factor," she said.

Epigenetic factors affect the way genes are expressed after DNA forms, the release noted. One environmental epigenetic factor is cigarette smoke.

"DNA is written in pen. Epigenetics is written in pencil," Kheradmand said. "If you have enough genes affected by epigenetic factors strung together, it can tip you over into lung damage and emphysema. The inflammation that drives emphysema could also drive cancer development, a testable hypothesis that we have begun to pursue."